In optical communications networks, optical transceiver modules are used to transmit and receive optical signals over optical fibers. On the transmit side of a transceiver module, a light source (e.g., a laser diode) generates amplitude modulated optical signals that represent data, which are received by an optics system of the transceiver module and focused by the optics system into an end of a transmit optical fiber. The signals are then transmitted over the transmit fiber to a receiver node of the network. On the receive side of the transceiver module, the optics system of the transceiver module receives optical signals output from an end of a receive optical fiber and focuses the optical signals onto an optical detector (e.g., a photodiode), which converts the optical signals into electrical signals. The electrical signals are then processed to recover the data contained in the electrical signals.
The transmit and receive fiber cables have connectors on their ends (e.g., LC connectors), that are adapted to mate with transmit and receive receptacles, respectively, formed in the transceiver module. A variety of optical transceiver module configurations are used in optical communications network. Some optical transceiver modules have multiple laser diodes on the transmit side and multiple photodiodes on the receive side for simultaneously transmitting multiple optical signals and receiving multiple optical signals, respectively. In these types of transceiver modules, the transmit fiber cables and the receive fiber cables have multiple transmit and multiple receive optical fibers, respectively. The transmit and receive fiber cables are typically ribbon cables having ends that are terminated in a connector module that is adapted to be plugged into a receptacle of the transceiver module.
Some optical transceiver modules have a single laser diode on the transmit side and a single photodiode on the receive side for simultaneously transmitting an optical signal and receiving an optical signal over transmit and receive fiber cables, respectively. Each of the cables has a single transmit and a single receive fiber, respectively. The ends of the transmit and receive cables have connectors on them that are adapted to plug into transmit and receive receptacles, respectively, formed in the transceiver module. These types of transceiver modules are often referred to as pluggable transceiver modules. Small form-factor pluggable (SFP) and SFP+ transceiver modules are examples of pluggable optical transceiver modules.
Typically, pluggable transceiver modules, such as the SFP and SFP+ transceiver modules, for example, are designed to be inserted into cages. The pluggable transceiver modules and the cages have locking features disposed on them that allow the transceiver modules to mate with an interlock with the cages. The pluggable transceiver modules typically include latch lock pins that are designed to be received in latch openings formed in the cages. In order to mate the pluggable transceiver module with the cage, the module is inserted into the cage and a latching mechanism is moved to a latching position to cause the latch lock pin on the transceiver module to be extended into the latch opening formed in the cage. In order to remove the transceiver module from the cage, the latching mechanism is moved to a de-latching position to cause the latch lock pin to be retracted from the latch opening, allowing the transceiver module to be pulled out of the cage.
FIG. 1 illustrates a side cross-sectional view of the transmit side of a known SFP optical transceiver module 2. The receive side of the module cannot be seen in the side cross-sectional view depicted. The transceiver module 2 includes a module connector 4 and a module receptacle 5 that are mated with each other and encased within a module casing 6. The module connector 4 includes a ferrule 7 that holds an end (not shown) of an optical fiber (not shown). The module receptacle 5 includes a module body 5a and an optical subassembly (OSA) 8. The OSA 8 includes an optics system 9, a laser diode 11, and other electrical components (not shown), such as a laser driver integrated circuit (IC) and a receiver IC (not shown), and another opto-electronic component, such as a photodiode (not shown), that operates as a light detector. In the example of the known SFP optical transceiver module 2 shown in FIG. 2, the laser diode 11 is a vertical cavity surface emitting laser diode (VCSEL). A rectangular heat sink structure 13 is disposed within the casing 6 and has a surface 13a that is in contact with the substrate of the VCSEL 11. The heat sink structure 13 typically made of a metal, such as copper or nickel-plated copper. Another surface 13b of the heat sink structure 13 is in contact with a non-metallic thermal pad 14, which is sandwiched between the surface 13b and the inner surfaces 6a of the module casing 6. The thermal pad 14 typically has a thermal conductivity of about 1 Watt per Kelvin-meter (W/K-m).
Heat generated by the VCSEL 11 is transferred through the heat sink structure 13 into the thermal pad 14, which then thermally couples the heat into the module casing 6. The heat spreads out in the casing 6 and is dissipated. A temperature sensor 15 is used to monitor the temperature of the module casing 6. The thermal pad 14 not only helps thermally, but also is necessary to remove additional space between the OSA 8 and the module body 5a caused by mechanical tolerance differences between these parts. When the OSA 8 is assembled to the module body 5a, mechanical tolerance differences between these parts often results in these parts not being properly aligned. The sandwiching of the thermal pad 14 between the surface 13b of the heat sink structure 13 and the inner surfaces 6a of the module casing 6 removes the additional space caused by mechanical tolerance differences. This, in turn, ensures proper mechanical alignment of the OSA 8 within the module body 5a. 
Switch manufacturers are typically required to ensure that the module casings of optical transceiver modules are maintained at around 70° Celsius (C). With current data rates of around 10 gigabits per second (Gbps), the maximum allowable temperature for the substrate of the VCSEL 11 before performance starts to degrade is about 85° C. This means that the thermal resistance between the module case and the VCSEL 11 is allowed to be relatively large as long as the maximum temperature difference between the substrate of the VCSEL 11 and the module casing 6 does not exceed 15° C. The heat sink structure 13 in combination with the thermal pad 14 is generally capable of achieving these goals. In such a configuration, the best temperature difference between the substrate of the VCSEL 11 and the module casing 6 achievable is around 5 to 10° C., depending on the thermal conductivity of the thermal pad 14 and the heat sink structure 13.
For current generation pluggable optical transceiver modules, it is clear that this solution and the resultant temperature drop that it achieves are more than sufficient to satisfy the thermal requirements for the VCSEL 11. However, as data rate demands increase, the amount of power consumed at both the transceiver module level and at the switch level also increase, which makes it more difficult to maintain the module casing 6 at around 70° C. Additionally, the higher data rates make it necessary to lower the maximum allowable temperature of the substrate of the VCSEL 11 to about 75° C., which reduces the amount of margin available for the temperature difference between the temperatures of the module casing 6 and the substrate of the VCSEL 11. Therefore, it is apparent that a new solution is needed that will be capable of overcoming these obstacles.
Accordingly, a need exists for a heat sink solution that is capable of meeting these thermal dissipation demands as laser diode data rates increase. A need also exists for a heat sink solution that is capable of meeting such thermal dissipation demands while also removing additional space caused by mechanical tolerance differences of parts in the same way in which the thermal pad 14 performs that function.